Polarization imaging

ABSTRACT

Methods of monitoring critical dimensions in a semiconductor fabrication process include capturing at least one image of a first structure that has an effect on the polarization state of light reflected therefrom. For at least some of the first structure images, a value is calculated indicative of intensity of light reflected from the first structure. A critical dimension of the first structure is obtained and correlated with the calculated value. At least one image of a subsequent structure is captured. A determination is made, based at least in part on the calculated value, of a critical dimension of the subsequent structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a U.S. national stage application claiming priorityunder 35 U.S.C. §371 to International Application Serial No.PCT/US10/24358, filed Feb. 17, 2010, which is a continuation-in part ofU.S. application Ser. No. 12/551,702, filed Sep. 1, 2009, now abandoned,which is a continuation of U.S. application Ser. No. 11/678,407, filedFeb. 23, 2007, now U.S. Pat. No. 7,586,607, which claims the benefit ofU.S. Provisional Application Ser. No. 60/793,858, filed Apr. 21, 2006and U.S. Provisional Application Ser. No. 60/844,297, filed Sep. 12,2006; and PCT/US10/24358 also claims the benefit of U.S. ProvisionalPatent Application Ser. No. 61/153,594 filed on Feb. 18, 2009; theteachings of all of which are incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to inspection and metrologytools for used to ensure quality and improve yield in semiconductordevice manufacturing processes.

BACKGROUND OF THE INVENTION

In lithographic semiconductor device fabrication processes, it isimperative that a stepper precisely focus an image of a reticle on asemiconductor substrate or wafer. Where the image of the reticle is outof focus, a state also known as defocus, the structures of the resultingsemiconductor device may be of the incorrect size and form. For example,the edges of the resulting structures may be relatively diffuse andindistinct, having rounded or undercut surfaces in lieu of a moredesired, often rectilinear geometry. This state of defocus often leadsto poor function and/or inoperability in the semiconductor device inquestion. Measurement of defocus is therefore an important means forallowing semiconductor device manufacturers to ensure that a stepperconsistently focuses a reticle image on a wafer, thereby enabling largerand more profitable yields from the manufacturing process.

Another problem common to the formation of semiconductor devices is thatof exposure defects. Where the exposure of a photo resist layer to lightfalls outside a range of acceptable light dosages, the features that areto be formed on the semiconductor substrate may be formed incorrectly.Accordingly, it is also important to identify exposure defects wherethey exist.

In addition to inspecting a substrate or wafer for exposure or defocusdefects, it is important to inspect substrates and wafers for process ormaterial related defects commonly referred to as “macro” defects. Macrodefects are often defined as chips, cracks, scratches, pits,delaminations, and/or particles that appear on a substrate that have adimension of about 0.5 u to 10 u in size. Such defects can easily causea failure in a semiconductor device and can significantly reduce theyield of a manufacturer of such devices. Note that the sizes of macrodefects may depart up or down from the size range stated above, whichmerely defines a nominal size of such defects.

Traditionally, macro defects have been inspected using dedicatedinspection systems that have not been able to readily or reliablyidentify the presence of exposure or defocus defects. Exposure anddefocus defects are usually identified using optical critical dimension(OCD) techniques on any of a number of precision metrology tools such asellipsometers, reflectometers and scatterometers. It would be desirableto combine the functions of identifying the presence of exposure anddefocus defects with inspection of substrates for macro defects whereinthe same optical system is used for both functions.

SUMMARY OF THE INVENTION

One embodiment of an inspection system for identifying defects on asubstrate includes a light source that directs light onto a substratethat is to be inspected. A first polarizing filter, or polarizer, ispositioned between the light source and the substrate. A secondpolarizing filter, or analyzer, is positioned between the substrate andan optical sensor that receives light reflected from the substrate. Thepolarizer and analyzer, are angularly arranged with respect to oneanother such that an image intensity of an image captured by the opticalsensor is at least partially correlated with the presence ofpolarization dependent defects on the substrate under test. Polarizationdependent defects include, among other things, defocus and exposuredefects. Defects having a main dimension of approximately the wavelengthof incident light or smaller that are not defocus or exposure defectsmay also be identified.

The light source may be of any useful type including, but not limitedto, a broadband incandescent light or a laser. Either of these lightsources may strobe and may be positioned to direct light on the surfaceof the substrate at any useful angle of incidence, including a normalangle of incidence. Lasers may be of a fixed, monochrome variety or maybe arranged to output light at several different nominal wavelengths.

Where strobe illumination is used, the strobe will flash on and off in asequence that at least partially correlates with the velocity at which asubstrate is moved with respect to the inspection system. This permitsthe inspection system to reliably capture images of the substrate at theappropriate locations.

The optical sensor or imager may be a monochrome charged capacitancedevice (CCD). In some instances, the optical sensor may be a colorimager of the Bayer type or a three-chip design. In yet other instances,one or more light source and/or color filters may be used in conjunctionwith a monochrome optical sensor to obtain color data from thesubstrate. Both area scan and line scan optical sensors may be used.

In addition to defocus and exposure defects, other types of defects maybe identified. These other defects may include pits, voids, chips,cracks, particles, and scratches.

Inspection systems according to the present invention are put intooperation by first arranging the light source to direct light onto thesubstrate. The first polarizing filter is positioned between the lightsource and the substrate and the optical sensor is placed to receivelight reflected from the substrate. The second polarizing filter isplaced between the substrate and the optical sensor and such that thefirst and second polarizing filters are at a selected relative anglewith respect to one another. The inspection system is then used tocapture images of the substrate and comparative data is generated fromthese images to identify the existence of exposure and/or defocusdefects on the substrate, if any. Arranging the polarizing filters tocapture the required images may involve rotating the first and secondpolarizing filters together to a desired inspection angle whilstmaintaining the selected relative angle therebetween.

Comparative data may be obtained by first generating a differentialimage for each captured image and then averaging pixel intensitydifferences of the respective differential images over the entiredifferential image to obtain an average image intensity for eachdifferential image. The average image intensity of each captured imageis evaluated with respect to a predetermined threshold to determine theexistence of at least one of exposure and defocus defects on thesubstrate, if any.

Calibration of the output of the optical sensor with respect to knownlevels of at least one of an exposure and a defocus defect in asubstrate is used to determine appropriate defocus and exposure defectlevels. In one embodiment calibration involves capturing a plurality ofimages of a calibration substrate, wherein each image is subject to aknown degree of defocus and exposure defects. As described above, adifferential image is generated for each captured image and the pixelintensity differences of the differential images are averaged over theentire differential image to obtain an average image intensity. Theaverage image intensity values for each captured image having a knowndegree of defocus and exposure defect are recorded. A user may selectany recorded average image intensity value indicative of a particulardegree or magnitude of defocus and/or exposure defects as a threshold,may interpolate between such recorded values or may simply use therecorded values as a starting point to which modifiers specific to theproduct are applied. It is entirely up to the user of an inspectionsystem to define suitable thresholds for defocus and/or exposuredefects.

Generating a differential image may involve averaging a plurality ofcaptured images on a pixel by pixel basis to obtain an averaged image.This averaged imaged is then subtracted from each captured image, on apixel by pixel basis to produce a differential image that can also bethought of as an array of pixel intensity values or as an array of pixelintensity value differences.

Inspection of a substrate for defocus and/or exposure defects may takesimultaneous with inspection for other defects such as pits, voids,chips, cracks, particles, and scratches. Alternatively, inspection forthese respective types of defects may take place successively or even ina time shifted manner, i.e. at times that are significantly separatedfrom one another.

In another embodiment of the present invention, image analysistechniques such as spatial pattern recognition (SPR) techniques may beused to analyze a differential image to identify the boundaries oflayers on a substrate. Note that layer boundaries such as theaforementioned ones may be part of layers that are an intentional partof the substrate or may be related to residues that are notintentionally part of the substrate, i.e. the layers may be contaminantsof one type or another.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of one embodiment of an imaging system of thepresent invention having a nominal angle of incidence other than 90degree.

FIG. 2 is a schematic view of one embodiment of an imaging system of thepresent invention having a nominal angle of incidence of substantially90 degree.

FIG. 3 is an illustration, in vector form, of the relative components ofreflected light reflected from a substrate.

FIG. 4 is a chart showing the relative components of reflected lightbefore being passed through an appropriately arranged analyzer.

FIG. 5 is a chart showing the relative components of reflected lightafter having been passed through an appropriately arranged analyzer.

FIG. 6 is a flow chart illustrating a method of setting up an inspectionsystem for inspection.

FIG. 7 is a flow chart illustrating a method of inspecting a substrate.

FIG. 8 is a schematic illustration of an array of integrated circuitdevices or chips formed on a semiconductor substrate.

FIG. 9 is a black and white or grayscale image of a test structureformed of a number of CD boxes having different critical dimensions.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof

The present invention involves a method and apparatus for determiningthe presence of exposure and defocus lithographic defects in asemiconductor substrate by measuring a change in the polarization oflight reflected from the surface of the substrate. To simplify thefollowing discussion, the term “defocus” will be used herein to denoteboth exposure and defocus defects, though it is to be understood that agiven substrate may suffer from the one defect or the other or both.Further, the term “defocus” shall be construed broadly to encompass anydefect or undesirable feature of a substrate under inspection that hascharacteristic results similar to exposure and/or defocus defects andwhich may be identified or otherwise characterized by the inspectionsystem of the present invention. In general, defocus defects arepolarization dependent features, that is, defocus defects will cause achange in the polarization state of light reflected therefrom, though itis understood by those skilled in the art that other aspects of defocusdefects may affect the nature and extent of such polarization changes.

As used herein, the term “substrate” shall be construed to include anymaterial or structure that may be inspected by the inspection system ofthe present invention. Specifically, the term “substrate” shall beconstrued to include, among other things, semiconductor wafers of anyconstruction or form or material, including, but not limited to, wholewafers, unpatterned wafers, patterned wafers, partially patternedwafers, broken wafers that are wholly or partially patterned, brokenwafers that are unpatterned, sawn wafers in any form and on any supportmechanism, including film frames, JEDEC trays, Auer boats, die in gel orwaffle packs, multi-chip modules often referred to as MCMs, etc. Theterms “substrate” and “wafer” may be used interchangeably herein.

The term “macro defect” as used herein will include all unintentionalfeatures that appear on a substrate that are essentially polarizationindependent. As noted above, macro defects are traditionally describedas being chips, cracks, pits, particles, scratches and the like. Notethat in some instances, the size of macro defects may approach thewavelength of the incident radiation being used for inspection purposes.In these instances, macro defects may have an effect on the polarizationstate of light reflected therefrom.

With reference to FIG. 1, one embodiment of an imaging system 8 includesan illuminator 10, a polarizer 12, an analyzer 14, and an optical sensor16. The illuminator 10 directs light along optical path P onto thepolarizer 12, which transmits substantially only light having apredetermined polarization angle. The light transmitted by the polarizer12 is then incident upon a substrate S. In one embodiment, the substrateS is a silicon wafer or a portion thereof having structures formedthereon. In some embodiments, these structures form one or moresemiconductor devices on the substrate S. Other mechanisms andstructures may also be formed on the substrates S.

As can be seen in FIG. 1, optical path P is at a non-normal angle ofincidence with respect to the substrate S. In some embodiments theilluminator 10, polarizer 12, analyzer 14, and optical sensor 16, aswell as other associated optical elements such as objectives and thelike, may be mounted so as to permit the angle of incidence of light onthe substrate S to be modified. Mounting mechanisms of the type thatwould be useful in modifying the angle of incidence of light in thesystem 8 are known to those skilled in the art and may include amounting plate(s) to which the optical elements of the system 8 aremounted, the mounting plate(s) being rotatable by a rotation means thatcan be one or more actuators. The angle of incidence may be essentiallyfixed (as shown) or may be modified for each product set-ups. Further,in some embodiments, the angle of incidence may be modified duringinspection, as needed.

The illuminator 10 may be of any useful type, including a broadbandwhite light, a laser having a fixed wavelength output, a laser arrangedto output multiple wavelengths, or a plurality of lasers arranged todirect light along the optical path P. The intensity required of theilluminator may vary depending on the application to which the system 8is directed. In some applications, a high intensity illumination isrequired and conversely, in others, relatively lower intensities arerequired. The illuminator 10 may be arranged to provide substantiallyconstant output or may be arranged to strobe so as to freeze the motionof a substrate S in the system 8, thereby allowing the rapid capture ofimages of the substrate S.

The light incident upon the substrate S is reflected therefrom and thisreflected light is incident upon an analyzer 14 which, being apolarizing optical element similar to polarizer 12, passes only thatlight having a predetermined polarization angle. What light passesthrough analyzer 14 is incident upon optical sensor 16, which capturesan image of the substrate S. Optical sensor 16 is in one embodiment atwo dimensional electronic optical sensor such as charge-coupled device(CCD), though any device capable of producing a two dimensional array ofpixel intensity values on a grayscale or color basis, such as a linescan or time delay integration imaging (TDI) device or a CMOS opticalsensor array, may be used. In one embodiment, the optical sensor 16 is amonochrome optical sensor wherein each pixel of the optical sensor's 2Darray of pixels registers a grey scale value of 0-256, which pixels,taken together represent an image of the substrate S. Where a monochromeoptical sensor is used, one or more color filters 18 may be positionedin the optical path P between the illuminator 10 and the optical sensor16 to pass only light within the range of wavelengths to which the colorfilter corresponds. In another embodiment, the optical sensor may be acolor optical sensor of the Bayer type or a three-chip color sensorhaving separate optical sensors, each dedicated to a separate color,e.g. one sensor for red light, one sensor of blue light, and one sensorfor green light.

Those skilled in the art will understand that the basic elements of thesystem 8 described herein above may be used in conjunction, or not, withother optical elements including, but not limited to optical filters,lenses, mirrors, retarders and modulators. One inspection system thatmay be adapted for carrying out the present invention is marketed byRudolph Technologies, Inc. of Flanders, N.J. under the trade nameWaferView™. Further, it is to be understood that the system 8 may bearranged to carry out multiple functions, these multiple functions beingcarried out in simultaneous or in temporally spaced arrangements. Forexample, the system 8 may be adapted to carry out an inspection formacro defects as well as an inspection for defocus defects. Further, thesystem 8 can be arranged to carry out an inspection for macro defectsfollowed by an inspection for defocus defects, or vice versa, or maycarry out both inspections simultaneously.

Color filters 18 may be used in the system 8 as shown schematically inFIG. 1. One or more color filters 18 may be placed between the polarizer12 and the substrate S, between the substrate S and the analyzer 14,between the illuminator 10 and the polarizer 12, or between the analyzer14 and the optical sensor 16. In one embodiment, the color filter 18 maybe a filter wheel of a type known in the art wherein one of a group ofcolor filters is affixed to a rotating wheel placed in the optical pathP such that the color filters 18 are selectively positioned across theoptical path P. In another embodiment, a removable filter holder may beplaced in the optical path P to permit different color filters to beplaced in the optical path P. In yet another embodiment, a fixed colorfilter may mounted in the optical path P. It is to be understood thatany filter media or mechanism suitable for selectively passing apredetermined wavelength or range of wavelengths may be used as a colorfilter.

In some embodiments, it is desirable to separate the output of theoptical sensor 16 with respect to predetermined color channels wherein“color channel” is defined as a predetermined wavelength or range ofwavelengths. Separation of color channels may be accomplished, assuggested above, by the use of color filters, by the use of coloroptical sensors that incorporate directly the ability to distinguishbetween respective color channels as do three-chip optical sensors andBayer optical sensors, or by using an illuminator 10 that outputs lightwithin a pre-selected range of wavelengths. It should be understood thatsome substrates S may be partially or wholly transmissive with respectto certain ranges of wavelengths or colors. By way of example only, agiven substrate S may transmit or destructively interfere a majority ofall incident blue light having wavelengths centered around 475 nm, butreflect a large portion of the red light having wavelengths centeredaround 700 nm that is incident on the substrate S. In this example, itmay be useful then to be able to use the signal output by the opticalsensor 16 that results from the red light incident on the optical sensor16. The use of data relating to a respective color channel will dependon what features are being examined in the inspection being undertakenwith the system 8. In some embodiments, certain semiconductorsubstrates, also known as products, will tend to have features thatreflect light in a known manner and accordingly, an inspection system 8may be set up specifically for a given product to optimize theinspection of the product.

FIG. 2 illustrates another embodiment of the present invention whereinan inspection system 30 has an illuminator 32 that directs light alongoptical path P though polarizer 36, filter 40 (optional), and beamsplitter 42 onto substrate S in a substantially normal arrangement.Light reflected from substrate S on path P is directed by beam splitter42 through filter 40 (optional) and analyzer 38 to optical sensor 34.Systems 8 and 30 are, aside from the presences of a beam splitter anddifferences in the angle of incidence, substantially similar. In thisembodiment, optical path P is substantially normal to the substrate S.

It has been observed that defocus defects will modify the reflectivityof a substrate S as defocus defects will modify the geometry of thestructures formed on a substrate S. Other factors that will modify thereflectivity of a substrate S are the properties of other film layersand the wavelength, polarization, and angle of light incident upon thesubstrate. Using an inspection system according to various embodimentsof the present invention, e.g. systems 8 or 30, it is possible todistinguish those reflectivity changes that result from defocus defectsand to do so in a rapid and reliable manner.

In general, light from an illuminator 10, 32 is polarized to apredetermined angle P by a polarizer 12, 36 and incident upon asubstrate S at a specified angle θ (theta).

Upon reflection, the substrate S will modify the polarization state ofthe incident light in relation to a number of characteristics thereofand specifically defocus. From this modification of the polarizationinformation concerning defocus defects in the substrate S can beobtained. The reflected light passes through analyzer 14 and is incidentupon optical sensor 16. The analyzer 14, when arranged as described inmore detail hereinbelow, helps ensure that the data obtained from theoptical sensor 16 includes information concerning both the amplitude andthe polarization change of the reflected light and particularlyinformation concerning the presence of defocus defects on the substrateS.

In one embodiment, the light incident upon the substrate S is linearlypolarized by polarizer 12, 36 as this often presents the simplestsolution to the inspection problem at hand. In other embodiments,incident light is elliptically or circularly polarized, as needed. Bysetting the analyzer 14, 38 at an angle A with respect to the polarizer12, 36, the light that reaches the optical sensor 16, 34 may bemodulated. The angle between the polarizer 12, 36 and the analyzer 14,38 is given as P-A.

Referring now to FIG. 3, in general, when light is reflected from thesubstrate S, portions of the incident light, E_(P), will be reflecteddifferently than other portions of the incident light. A portion of theincident light E_(P) is reflected from polarization independent featureson the substrate S surface without any significant modification in itspolarization as illustrated in FIG. 3 at E₁ and E₂. Some examples ofsuch features that may be found on a semiconductor substrate S include,but are not limited to, bright and dark appearing defects in thesubstrate S such as chips, cracks, scratches, pits, voids, andparticles. Another portion of the reflected incident light, E₃, isreflected from polarization dependent features formed on the substrate Sthat are arranged in such a way that they will modify the polarizationof the incident light. Some examples of the polarization dependentfeatures or structures found on a semiconductor substrate S include, butare not limited to, line structures, conductors, interconnects, vias andstreets. Yet another portion of the incident light reflected from thesurface of the substrate S is reflected by features or structures thatmodify the polarization of the reflected light and which are alsosubject to defocus defects. This light, E₄, has a polarization statethat differs from E₃. Structures that reflect light E₄ may bestructurally similar or identical to the nominal structures from whichlight E₃ described above is reflected except for the fact that they aresubject to defocus defects, the magnitude of which will affect theintensity of light E₄. One example of a structure that is subject todefocus defects are test structures 14 shown in FIG. 8.

FIG. 8 is a schematic illustration of a portion of a substrate, in thisinstance a semiconductor wafer W, having a number of integrated circuit(“IC”) devices 70 formed thereon. The IC devices 70 are laid out in agrid pattern having gaps therebetween known as “streets” 72. In thestreets 72 are formed a number of test structures 74. On each of the ICdevices 70 there is also formed a structure 76 that forms a part of theactive circuit of an IC device 70 and which is subject to defocusdefects. It should be noted that in some instances, substantially theentire IC device 70, or at least significant portions thereof, mayconstitute one or more structures 76 that are subject to defocusdefects. Test structures 74 and structures 76 are often formed so as tobe similar to one another. For example, if the structure 76 that is partof the active circuit of an IC device 70 has a series of linearstructures formed therein, a test structure 74 will be formed in muchthe same way so that characteristics of the test structure 74 will beindicative of characteristics of the structure 76.

In some embodiments, test structures 74 comprise a series of boxes 78each of which has a periodic structure formed therein. FIG. 9 is aphotograph of an exemplary test structure 74 having a number of discreteareas 78. Each of the discrete areas 78 has a nominally differentcritical dimension, i.e. has a periodic structure having a specifiedpitch or critical dimension. Note that the net effect of a teststructure 74 is to provide some of the information normally onlyavailable from an FEM wafer on each product wafer. Because periodicstructures at certain critical dimensions will have an effect on thepolarization state of light reflected from the periodic structures, thetest structure 74 is useful for assessing changes in the operation of astepper that performs pattern forming steps in the photolithographicprocess. Further, where the individual areas 78 of the test structure 74are sufficiently similar to the structures 76 located in the active areaof the IC device 70 and presuming that additional process variationsthat may affect the change of the polarization state of incident lightsuch as the relative thickness or material properties of layers formingthe substrate W on which the IC devices 70 are formed, one may obtain ageneral indication of the value of a critical dimension of the structure76.

As changes in polarization state of light incident upon the surface of asubstrate may result in changes in the intensity of the reflected light,in some instances, an average grayscale value for a given area 78 maycorrelate directly and unambiguously to the area's critical dimension,which correlation may then be used to identify a critical dimension of astructure 76 based on a grayscale value for the structure 76. While acritical dimension value for a structure 76 obtained in this manner maynot be strictly accurate, the use of a test structure 74 havingrepresentations of multiple critical dimensions to assess criticaldimensions of structures 76 on a substrate is useful for obtaininginformation of the magnitude of any changes in critical dimension in theIC devices 70. For example, grayscale information indicative of criticaldimensions may be used to identify a magnitude of change in criticaldimension in addition to or in lieu of assessing an “absolute” criticaldimension by measuring the magnitude of the difference between agrayscale value of a selected test structure 76 or an individual area 78thereof and that of a subsequently inspected or imaged structure 76 ortest structure 74.

Referring to FIG. 4, it can be seen that for light that has not passedthrough the analyzer 14, 38, there is little contrast between theintensity of reflected light E₃ and E₄. As can be appreciated, it isdifficult to identify defocus defects under these circumstances.However, once the reflected light E₁, E₂, E₃, and E₄ is passed through aproperly arranged analyzer 14, 38, to obtain light signals E′₁, E′₂,E′₃, and E′₄, the contrast level between light signals E′₃, and E′₄ issufficient to obtain useful information concerning the presence ofdefocus defects. See FIG. 5.

In one embodiment, the angle P-A between the polarizer 12 and theanalyzer 14 is determined experimentally. Referring now to FIG. 6, witha test substrate S positioned in the system 8 for inspection (step 50),the illuminator 10, which is in one embodiment a strobe illuminator, isset to a predetermined illumination level (step 52) that is preferablynear the top end of its intensity output, but may be of any suitableintensity. Light E_(P) from the illuminator or light source 10 isdirected through the polarizer 12 and onto the substrate S. Next, thepolarizer 12 is set to an angle P (step 54). In one embodiment, thepolarizer 12 is angularly oriented substantially perpendicular to anylinear structures present on the substrate S. As will be understood,where the substrate S is a semiconductor wafer having semiconductordevices formed thereon (in any state of completeness), such structurestypically, but not always, have significant linear features. Reflectedlight (E₁, E₂, E₃, and E₄) is passed through the analyzer 14 to obtainlight signals E′₁, E′₂, E′₃, and E′₄ on optical sensor 16. In instanceswhere there is no discernible orientation to linear structures on asubstrate S or where the substrate S has no linear structures formedthereon, an arbitrary angle P may be chosen for polarizer 12.

The analyzer 14 is next rotated to an angle A (step 56) such thatsufficient illumination reaches the optical sensor 16 to permitinspection of the substrate S for macro or polarization independentdefects as described in U.S. Pat. Nos. 6,324,298, 6,487,307 and6,826,298, which are owned jointly herewith and which are herebyincorporated by reference. Note that the analyzer 14 will be consideredto be at the correct angle A when the illumination not only allows forinspection of the substrate S for defects, but also does so with asignal to noise ratio that permits for an inspection of sufficientquality to satisfy an end user of the system 8 that significant errorssuch as false positives and missed defects do not occur in theinspection. The signal to noise ratio of the system 8 is determined byanalyzing the output of the optical sensor 16 in a known manner.

Once the angles P and A at which the polarizer and analyzer arepositioned are known, the polarizer and analyzer are rotated togetherthrough a series of inspection angles (step 58), keeping the relativeangle between the polarizer and analyzer (P-A) substantially constant,to a desired angular position with respect to the substrate S that willprovide the necessary contrast between light signals E′₃ and E′₄ asdescribed above. During rotation of the polarizer 12 and the analyzer14, the intensity of light incident upon the optical sensor 16 isrecorded. Light intensities at the optical sensor 16 are recorded foreach of the inspection angles or positions through which the polarizer12 and analyzer 14 are rotated. In one embodiment, the polarizer andanalyzer 14 are rotated in a more or less continuous manner and theposition of the polarizer and analyzer and the light intensity presentat the optical sensor 16 are recorded in small increments of rotation ofthe polarizer and analyzer. Analysis of the data obtained duringrotation of the polarizer and analyzer is performed to identify theoptimal inspection angle or position for the polarizer and analyzer withrespect to the contrast between reflected light E′₃ and E′₄.

The process of identifying an optimal setting for angles P and A may bemanual, wherein a user of the system 8 rotates the polarizer 12 andanalyzer 14 through a selected range of angles while the optical sensor16 records image data which is processed by a computer C of a suitabletype to determine the optimal angle P-A. Alternatively, and preferably,the polarizer 12 and analyzer 14 will be automated such that theaforementioned computer can control the rotation thereof while itrecords data from the optical sensor 16 at various angles P-A.Automation of polarizers and analyzers is well known to those skilled inthe art. As suggested above, the process of determining an optimal angleP-A angle between the polarizer and analyzer may require multipleiterations, both before and after the steps that are describedimmediately hereinbelow. For example, once an entire calibration/set-upprocess is completed, it may be useful to run the entirecalibration/set-up process multiple additional times to determinewhether the resulting system set-up is optimal.

Once the system 8 has been appropriately set up as described inconjunction with FIG. 6, inspection for defocus defects and if desired,other defects, may take place. First, however, the system 8 must becalibrated. Calibration is preferably carried out using a focus exposurematrix (FEM) wafer. An FEM is a substrate on which a number of patternsor structures have been formed, each with a different focal position andexposure (exposure) time. An FEM is commonly created as part ofcalibrating a photolithography tool for the production of semiconductordevices. The FEM embodies structural changes in the patterns orstructures formed on the substrate S that result from changes in focalposition and exposure. Defocus and exposure data obtained from the FEMis used as a comparator during inspection of substrate S. Note that thepatterns formed on the FEM may be different than those formed on thesubstrates S under test, but are preferably the same.

As the method of obtaining defocus data is substantially the same forcalibration purposes as it is for inspection purposes, the method ofobtaining defocus data for calibration purposes will be described aspart of the inspection process.

Differences between calibration and inspection procedures will be notedwhere appropriate.

During inspection, a substrate S of the type that is to be inspected isobtained and placed on a wafer support or top plate (not shown) thatmoves the substrate S relative to the optics of the inspection system 8in a known manner. Substrates S (product or FEM) are in some embodimentsinspected piecewise. In one embodiment wherein the substrate S is awafer on which semiconductor devices are formed, the inspection of thesubstrate S is carried out on a die level basis, that is, images of theindividual die on the substrate S are imaged by sensor 16 and thoseimages are processed as described hereinbelow. In other embodiments,inspection is carried out on a field of view basis. The optics of asystem 8 will be arranged to capture images of a field of view whosesize may differ from that of an individual die or that of an individualstepper shot. Where the field of view of a system 8 is smaller than anindividual die, multiple fields of view may be stitched together tocreate composite images of individual die. The same stitching techniquemay be used to form a composite image of an entire stepper shot. It isto be understood that stitching images to form a composite image is atechnique that is well known in the art.

In other embodiments where the field of view is larger than anindividual die or larger than a stepper shot, the resulting images maybe cropped to show one or more die or stepper shots. It is generallyuseful to not crop a larger image so as to include multiple die fromseparate stepper shots as those die created by a first stepper shot maybe acceptable while those die created by a second stepper shot may bedefective. Cropping an image is a technique well known to those skilledin the art.

In yet other embodiments, inspection of the substrate S is accomplishedby first capturing an image of an entire substrate S. Where thesubstrate S is relatively small, this may be capture using a system 8operating on an area scan principle. Where the substrate is larger thanthe field of view of an area scan system 8, multiple images of thesubstrate S may be obtained and stitched together as suggested above.

Stitching may be used in conjunction with line scan as well as area scantype inspection systems 8 as will be readily appreciated by thoseskilled in the art.

Once the appropriate polarizer/analyzer angle P-A is obtained asdescribed above, images of individual die on the substrate S arecaptured by the optical sensor 16 (step 60). See FIG. 7. The calibrationand inspection processes described hereinbelow will be described astaking place on a die by die basis, though it is to be understood thatother bases may be used. The die on the substrate S that are imaged maybe selected by a user who determines that the die are sufficiently freeof defects such as chips, cracks, pits, color variation, particles orthe like to form a model. This determination is entirely up to the userof the system 8 and can vary greatly depending on the nature andintended use of the product on the substrate S. For example, a userinspecting substrates S having semiconductor devices formed thereon thatare intended for use in pacemakers would likely impose very stringentstandards concerning the number of defects on a die that is to be usedfor the purpose of creating a model. Conversely, a user inspectingsubstrates S having identical semiconductor devices formed thereon thatare intended for use in an inexpensive and essentially disposableconsumer product would likely be willing to accept a higher number ofdefects in die that are to be used for the creation of a model. Inshort, it is up to the judgment of a user of a system 8 as to whatdefines a “good” die for the purposes of creating a model. While it isenvisioned that images of all “good” die on a substrate S may beobtained for the purpose of creating a model, it is typically the casethat only a statistically significant number of ‘good’ die; this numberis generally less than the total number of ‘good’ die and may be on theorder of about 10-15. At a minimum, die having no more than a modestnumber of random, relatively unobtrusive defects should be chosen assmall, random defects will not likely not have a significant effect oninspection if a statistically significant sample of such die aresampled, it being understood that large, non-random defects will be morelikely to skew an inspection process.

Automated methods may also be used to obtain a useful model. Forexample, a computer C that controls the system 8 may randomly select astatistically significant number of die and capture images thereof Theseimages are then used to form a model which is used to inspect theindividual images that formed the model. Where a selected die is foundto be defective under the user-selected criteria, the defective imagewill be replaced by an image of another randomly selected die. Thisprocess will be iterated until a suitable model is created. Note thatmodels created manually or automatically may remain static, i.e. willnot change over time, or may be modified over time by adding new, gooddie to the model as inspection progresses.

As the term “model” may have different meanings to different persons ofskill in the art, it should be made clear that as used herein, the terms“golden die” or “golden reference” are used to describe an image whoseconstituent pixels have intensity values obtained by summing thecorresponding pixel values of a number of die and obtaining an averageof those values. Accordingly, the golden die is simply an image ofaveraged die. The term “model” is broader than the terms “golden die” or“golden reference” and in some instances, will not incorporate or usegolden die or golden reference information.

A golden die is used in one embodiment of defocus inspection (step 62).

Similarly, golden die may form at least part of the basis for a modelused in macro defect inspection. Generally, however, models used inmacro defect inspection move beyond a simple golden die by definingpixel intensity thresholds for each pixel in an image. In macro defectinspection, if, upon evaluation, pixel intensity values are found tofall outside of the range defined by the thresholds, those pixels aredeemed to be defective. The thresholds themselves may be as simple as astandard deviation calculated from a golden die, but more often includeweighting factors that take into consideration various features,variations and characteristics of a substrate S and the user definedcriteria that apply to the product formed thereon. It is to beunderstood that models used for defect inspection may be formed inmyriad ways and may or may not be based on a golden die in any way, theonly requirement for macro defect inspection models being that theresulting inspection yields results that are satisfactory to the user ofthe system 8. Where macro defect inspection is to be carried out, asuitable model for such inspection may be obtained (step 64) at more orless the same time that a golden die is generated. As indicated by arrow65, the formation of a model may in some instances use golden dieinformation. Once a model has been created, subsequently captured imagesare compared with the model to identify defects (step 72).

The golden die image obtained in the previous step is used to remove thebackground of images captured during inspection for defocus defects,resulting in what is referred to as a differential image (step 66). Thedifferential image consists of the differences between individualcorresponding pixel values of the golden die image and an image of a dieunder inspection. The pixel intensity values that make up thedifferential image, which can be positive, negative or zero, are summedand averaged over the entire differential image (step 68). The resultingaveraged values are then compared with similar averages obtained frominspecting an FEM to determine whether the averaged values cross apredetermined threshold set by a user of the system. In some embodimentsit is possible that FEM-derived differential image average values may bedirectly compared with inspection derived differential image averagevalues to determine whether an unacceptable level of defocus defects arepresent in a die.

As is understood by those skilled in the art, the polarizer 12 andanalyzer 14 may be arranged angularly with respect to one another so asto prevent the passage of substantially all light or to permit thepassage of substantially all light. In one embodiment of the presentinvention, the polarizer 12 and analyzer 14 are arranged angularly withrespect to one another so as to prevent the passage of substantially alllight E₁ and E₂ therethrough. In this embodiment, and where thesubstrate S did not affect the polarization state of the reflectedlight, the optical sensor 16 would register substantially no image.However, since the presence of features that modify polarity aregenerally present on the substrate S and because at least some degree ofdefocus defects are typically present, light E₃ and E₄ will be incidentupon the optical sensor 16.

The angular positioning of the polarizer 12 with respect to the analyzer14 will most often depend on the nature of the substrate S beinginspected, though other factors may be used, including, but not limitedto the nature of the light source 12, physical properties of the opticalsystem and the like. In one embodiment, the polarization angle of thepolarizer 12 is about 45 degrees to the linear structures present on thesubstrate S being inspected. Accordingly, it is to be recognized that insome embodiments, the polarization angle of the analyzer 14 may varydepending on the nature of the substrate being inspected.

In some embodiments, a multiple scan inspection will be used todetermine the presence of defects on a substrate S. In one embodiment, afirst pass is undertaken with the polarizer 12 and analyzer 14 in asetting that passes insufficient light for macro defect inspection. Thisfirst pass is intended only to determining whether defocus or exposuredefects exist in the imaged area of the substrate, generally one or moredie or stepper shots. A second pass involves finding macro defects suchas chips, cracks, particles, voids and scratches and is undertaken withthe polarizer 12 and analyzer 14 arranged in a manner that allows agreater amount of light to pass therethrough.

In another embodiment, the system 8 may be used to detect changes in thethickness of, or the presence of, thin films on a substrate. In someinstances, unwanted residual films will remain on all or portions of asubstrate S after a processing step. Where properly arranged,differential images of a substrate 8 will identify the location andextent of residual films.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges can be made in form and detail without departing from the spiritand scope of the present invention.

CONCLUSION

Although specific embodiments of the present invention have beenillustrated and described herein, it will be appreciated by those ofordinary skill in the art that any arrangement that is calculated toachieve the same purpose may be substituted for the specific embodimentsshown. Many adaptations of the invention will be apparent to those ofordinary skill in the art. Accordingly, this application is intended tocover any adaptations or variations of the invention. It is manifestlyintended that this invention be limited only by the following claims andequivalents thereof.

What is claimed is:
 1. A method of monitoring critical dimensions in asemiconductor fabrication process comprising: a. capturing at least oneimage of a first structure having a geometry which has an effect on thepolarization state of light reflected therefrom; b. generating adifferential image for each captured image; c. averaging pixel intensitydifferences of the respective differential images over the entiredifferential image to obtain an average image intensity value for eachdifferential image; d. obtaining a critical dimension of the firststructure; e. correlating a critical dimension of the first structurewith the average image intensity value; f. capturing at least one imageof a subsequent structure; and, g. determining, based at least in parton the average image intensity value, a critical dimension of thesubsequent structure.
 2. The method of claim 1 further comprising: a.capturing at least one image of a second structure having a geometrywhich has an effect on the polarization state of light reflectedtherefrom; b. calculating, for each of the at least one images of thesecond structure a value that is indicative of an intensity of the lightreflected from the second structure; c. obtaining a critical dimensionof the second structure; d. correlating a critical dimension of thesecond structure with the calculated value; e. capturing at least oneimage of a subsequent structure; and, f. determining, based at least inpart on the calculated values of the first and second structures, acritical dimension of the subsequent structure.
 3. The method of claim 2further comprising: a. capturing at least one image of at least oneadditional structure having a geometry which has an effect on thepolarization state of light reflected therefrom; b. calculating, foreach of the at least one images of the at least one additional structurea value that is indicative of an intensity of the light reflected fromthe at least one additional structure; c. obtaining a critical dimensionof the at least one additional structure; d. correlating a criticaldimension of the at least one additional structure with the calculatedvalue; e. capturing at least one image of a subsequent structure; and,f. determining, based at least in part on the calculated values of thefirst, second and at least one additional structures, a criticaldimension of the subsequent structure.
 4. The method of claim 3 whereinthe first, second and additional structures are entire IC devices. 5.The method of claim 3 wherein at least one of the first, second andadditional structures is a selected portion of an active circuit of anIC device.
 6. The method of claim 1 wherein the first structure is apart of an active portion of an integrated circuit device.
 7. The methodof claim 1 wherein the critical dimension of the subsequent structure isan absolute value.
 8. The method of claim 1 wherein the criticaldimension of the subsequent structure is a differential value.
 9. Themethod of claim 1, wherein determining the critical dimension of thesubsequent structure is further based on correlating the criticaldimension of the first structure with the average image intensity value.10. A substrate inspection system comprising: an optical sensorpositioned to capture an image of a first structure having a geometrywhich has an effect on the polarization state reflected therefrom and animage of a subsequent structure; a computer coupled with the opticalsensor to receive the image of the first structure in the image of thesubsequent structure, the computer having instructions that, whenimplemented, generate a differential image for the image of the firststructure, average pixel intensity differences of the differential imageover the entire differential image to obtain an average image intensityvalue, obtain a critical dimension of the first structure, correlate acritical dimension of the first structure with the average imageintensity value and determine, based at least in part on the averageimage intensity value, a critical dimension of the subsequent structure.11. The system of claim 10 wherein the first structure is part of anactive portion of an integrated circuit device.
 12. The system of claim10, wherein the critical dimension of subsequent structure is anabsolute value.
 13. The system of claim 10 wherein the criticaldimension of the subsequent structure is a differential value.